An a.c. ignition system is known from the related art, e.g., in the form of the circuit illustrated in FIG. 3. An ignition transistor designed as an IGBT (insulated gate bipolar transistor) is driven by a control circuit A. In addition to an operational amplifier (difference amplifier), a current detecting device and a driver circuit (referred to below simply as a driver), this control circuit includes a timing element whose output signal is sent to the difference amplifier after being AND-ed with an ignition control signal. The mode of operation of this circuit is based on the fact that the ignition transistor is switched off when the voltage drop at a shunt resistor exceeds a certain value. The ignition transistor is then activated again after a certain period of time defined by the timing element. Using a timing element should achieve the result that the starting operation of the ignition transistor falls in a certain time window in which a diode arranged between the collector and emitter of the ignition transistor is active. In this case, the activation operation has practically no effect on the properties of the circuit.
However, this yields the disadvantage that the time window can shift due to tolerance or aging of the ignition coil and spark plug, i.e., the time-critical components, due to a different oscillation characteristics of the circuit in secondary idling and during sparking. If the control signal of the ignition transistor no longer falls in the active phase of diode D, so there are starting current peaks with each switching operation, the total power loss of the circuit is increased, and the available igniting voltage is also reduced.